1. Field of the Invention
The invention relates to a composite capacitive sensor device including a probe having an electrode with a flat surface and including a coaxial cable for connecting said probe to measurement means.
The invention is applied particularly in the aircraft industry for the dynamic measurement of the speed of rotation of rotor blades in the casing of a turbo engine and for the measurement of the clearance existing between the rotor blade tips and this casing.
2. Description of the Prior Art
A capacitive sensor for a turbo engine is already known from the Patent EP 0246576. This sensor comprises a metal probe body having two different inner diameters. The part having the larger inner width is disposed at the end of the probe and accommodates a flat disc-shaped electrode having a first surface which should be disposed opposite the rotor blade tips of the turbo engine and having its other surface connected to the central conductor of a coaxial cable. This coaxial cable is surrounded by a metal sleeve fixed to the small-diameter part of the probe body. In the large-diameter part the electrode is insulated from the probe body by a mineral insulator. The coaxial cable further comprises a guard electrode extended by a guard ring disposed around the electrode in the large-diameter part.
The materials and welding means of the various parts of the known sensor probe allow the probe to be used at a maximum temperature of 750.degree. C. To this end, the metal parts of the coaxial cable are made of Inconel and are insulated from one another by a mineral insulator, the electrode is made of platinum and is insulated from the probe body by an aluminum oxide, and the elements formed by the probe body and the sleeve on the one hand and by the electrode and the central conductor on the other hand are joined to one another by means of a solder material. Such joints also connect the guard electrode to the coaxial cable and the guard ring to the probe body.
It is an object of the present invention to provide such a capacitive sensor device suitable for use in a turbo machine referred to as a combustion chamber, which also comprises rotor blades, i.e. a rotor, in a stationary part, or stator, where the temperature at which the known sensor is used is of the order of 1300.degree. C. and the pressure is of the order of 40 bar.
It is another object of the present invention to provide a device whose electrode surface has a diameter which is as large as possible without the other specifications being affected, since the signal obtained for the capacitance measurement is proportional to the area of the conductive surface.